1. Field of the Invention
The present invention generally relates to an oligofluorene compound and an organic EL element using same.
2. Description of the Related Art
Organic electroluminescence elements (referred to hereinbelow as organic EL elements) are presently actively studied for use in planar light-emitting sources and thin display panels.
An organic EL element is typically composed of two electrodes and a multilayer thin film of an organic amorphous charge transport material sandwiched between the two electrodes. The drive principle of the organic EL element resides in the excitation of molecules contained in the thin film by injecting carriers, namely holes and electrons, from the electrodes into the thin film and causing the recombination of the carriers. The excited molecules then decay. Light emission occurs in this decay process and the emitted light passes to the outside of the element.
Light emission induced by voltage application to an anthracene single crystal (W. Helfrich Phys. Rev. Lett. 14, 229 (1964)) and light emission from a two-layer thin film of an organic amorphous charge transport material discovered by Tang et al. (Appl, Phys. Lett. 51, 913 (1987)) were reported a long time ago. Light emission from a double hetero structure (three-layer structure) composed of a hole transport layer, a light-emitting layer, and an electron transport layer disclosed in M. A. Baldo et al. Appl. Phys. Lett. 75, 4 (1999) has recently been suggested.
As another example, fluorescent light-emitting materials that use only a singlet exciton component and phosphorescent light-emitting materials using both singlet excitons and triplet excitons as a light emission source have been suggested as light-emitting materials. According to a spin selection rule, excitons generated by recombination of carriers are generated at a ratio of singlet excitons to triplet excitons of 1:3. The phosphorescent light-emitting materials can use both the generated singlet excitons and the generated triplet excitons as light emission sources. Therefore, the theoretical internal quantum efficiency of phosphorescence light-emitting materials is 100%, which is highly desirable for high-efficiency organic EL elements.
As described hereinabove, where a phosphorescent light-emitting material is used as a constituent material of an organic EL element, the efficiency in principle can be increased over that of other conventional fluorescent light-emitting materials. However, a sufficiently high light emission efficiency presently cannot be realized. For example, an organic EL element has been disclosed (Chihaya Adachi et al. Appl. Phys. Lett. vol. 78, 1622 (2001)) that uses (Btp)2Ir(acac), which uses a phosphorescent light-emitting material as a guest material and a well-known compound CBP as a host material. According to the disclosure, the maximum external quantum yield obtained by the combination of CBP and (Btp)2Ir(acac) is about 7%. Therefore, there remains a need for combinations of host materials and guest materials (dopant materials) that are capable of increasing the light emission efficiency.
Furthermore, the conventional design and selection of host materials for use with the phosphorescent light-emitting dopants have often been focused on obtaining a sufficiently high triplet energy, so as to inhibit a reverse energy transition from the triplet energy of a dopant to the triplet energy of a host. However, the effects of other physical parameters of host materials have not as yet been sufficiently studied.
Accordingly, the efficiency and decrease in voltage of organic EL elements are presently insufficient, and there remains a need for organic EL elements that have better light emission efficiency and can be driven by a lower voltage. Further, service life of organic EL elements is presently also insufficient, and organic EL elements with a longer service life are needed.